IEEE Geoscience and Remote Sensing Magazine - June 2020 - 60
FIGURE 17. A subimage area from Google Earth.
RECENT DEVELOPMENTS AND FUTURE TRENDS
As stated in previous sections, conventional ATI-SAR systems can measure only the velocity of moving targets in the
radial direction. However, when vehicles are moving on
roads or ships with linear wake features are observed, the
target moving direction can be determined. From both the
inferred moving direction and the evaluated radial velocity,
the full-velocity vector can be estimated [12]. In this case,
the use of road maps or linear wake features-recognition
methods is required. The use of external information, i.e.,
road maps, can also contribute to the estimation refinement of the radial velocity of vehicles. In fact, a refinement
of the radial velocity can be made in such a way that the
azimuth displacement (12) repositions the target exactly on
the road.
In the absence of moving-direction information, estimating the full-velocity vector requires more receiving
channels. Technological advances of space-based SAR, especially with regard to increased aperture sizes and additional receiver channels, have been incremental in the last
decades because of prohibitive costs.
A dual-beam airborne ATI-SAR system has been deployed. It has four antennas for each pair in the forward
and backward squint directions [10], [56]. Recently, a spaceborne, K-band dual-beam ATI system was proposed [57].
Other techniques use multilook/sublook processing in the
azimuth direction, applied to the range-compressed raw data
acquired by the fore and aft antennas, yielding four complex
subimages. By exploiting the diversity in azimuth-looking
directions, the direction of the velocity vector can be estimated [58]-[60].
Technological advances of space-based SAR, especially
in spacecraft with more than two parallel receive channels,
are, however, expected to materialize in next-generation
SAR systems. Only two channels will allow for full applications of optimum signal processing techniques based on
STAP with its considerably increased GMTI performance
60
[24], [47]. The background clutter will compromise less
STAP-based velocity determination, as it is suppressed prior
to the estimation. A first experimental proof of concept was
implemented on RDS-2 while in orbit in the form of cyclic
antenna aperture toggling and switching to create additional phase centers in a time-multiplexed fashion. Despite a reduced unambiguous pulse-repetition frequency (PRF) and
SNR, this four-channel mode (dubbed MODEX-2) clearly
confirmed the improvement regarding target parameter estimation [24], [47], [61].
Because of the requirement of a relatively high PRF to ensure high correlation among the channels, classical spacebased ATI-SAR systems permit only a narrow swath width
on the order of tens of kilometers. One predominant defense and security-driven operational requirement is large
coverage on the order of hundreds of kilometers. This is
currently being achieved via ScanSAR operation, which reduces the resolution inversely proportional to the increase
of the swath width. Hence, it is now only operationally
applied for maritime domain awareness [62]. Combining
ScanSAR imaging with ATI-SAR capability enhances the
detection of smaller ships in higher-clutter backgrounds,
i.e., high sea states or ice-ridden waters [63].
In contrast, for land GMTI applications, the drop in resolution and corresponding SNR when using ScanSAR is not
tolerable due to the much smaller target sizes. Here, a relatively new imaging capacity may build the foundation for
improved space-based GMTI. The concept of high-resolution
wide swath (HRWS) requires multiple along-track apertures
to keep the resolution common for strip map SAR (meters)
but increase the swath to the required hundreds of kilometers. It is likely that HRWS potentiality is one of the distinct
features for the next generation of SAR spacecraft, as the resulting imagery satisfies the partly contrary requirements of
various user groups, i.e., the monitoring of vast areas with a
simultaneous zoom-in capability for recognition and classification. Thus, HRWS will inevitably also push the development and operational utility of ATI applications, e.g., [64].
An alternative way to improve ATI-SAR performance is
based on the fact that the velocity estimation accuracy is inversely proportional to the SAR aperture length (and therefore, the corresponding baseline); see (24). Deploying larger
structures, i.e., a 100-m-long extractable boom in the flight
direction (similar to the SRTM) would permit, for instance,
the use of minimal redundant arrays to improve GMTI performance significantly [65]. If launching such large structures turns out to be impractical, one could exploit bistatic
SAR configurations to enhance the GMTI aptitude of imaging radars. In bistatic SAR, the transmitter and receiver are
located on different platforms. One example of this kind is
the TanDEM-X satellite, a clone of TerraSAR-X, launched in
2010 to measure the surface height and improve the topographic maps of Earth. This is accomplished by a much longer and variably adjustable baseline between the antenna
phase centers ranging from hundreds of meters to kilometers. Exploiting these extremely large but sparse antennas
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE
JUNE 2020
IEEE Geoscience and Remote Sensing Magazine - June 2020
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